Compact, portable, efficient, and low-cost spectrometers are of high interest in biological and environmental sensing. A conventional spectrometer generally has the following parts: a collimator; a wavelength dispersive medium; a collector; and a detector.
In conventional spectrometers, gratings are commonly used as wavelength dispersive media and can separate wavelength channels of a spatially coherent incident beam very well. However, a spatially incoherent beam consisting of multiple spatial modes results in the spatial overlap of multiple wavelength channels in the output plane of the grating. To avoid this problem, a collimator which is composed of a narrow slit and a lens (or a concave mirror) is placed in front of the grating. A main drawback of this arrangement is the low throughput as the slit blocks most of the input power. Increasing the slit width to improve the throughput results in less resolution. This trade-off between resolution and throughput is another disadvantage of a conventional spectrometer.
To improve the efficiency of the spectrometer and to make the spectrometer more compact, a Fourier-transform volume holographic (FTVH) spectrometer integrates the collimator (the slit and lens in front of the grating) and the grating into a spherical beam volume hologram (SBVH) recorded in a holographic material by the interference of a plane wave and a spherical beam. When the SBVH is read by a plane wave from the direction of the recording spherical beam, the diffracted beam has a crescent shape due to partial Bragg matching. The position of the crescent at the back face of the volume hologram depends both on the reading wavelength and on the direction of the reading plane wave. This effect results in considerable crosstalk between different incident wavelength channels when the reading beam is spatially incoherent (e.g., several incident angles) with multiple wavelengths.
To solve this ambiguity, a Fourier-transform lens (that plays the same role as the collector in conventional spectrometers) is placed behind the hologram so that all diffracted crescents corresponding to different incident angles but the same incident wavelength overlap at the same location in the Fourier plane. Therefore, the position of the Fourier spectrum of the crescents only depends on the incident wavelength, which is highly desirable for diffuse source spectroscopy. One advantage of the FTVH spectrometer is that the diffuse source can be placed right in front of the SBVH without any input coupling, which reduces the alignment requirements. Further, since no slits and lens are required in front of the hologram, the FTVH spectrometer is more compact than the conventional ones. However, the Fourier-transform lens behind the hologram is still essential for this FTVH spectrometer which may be a disadvantage in some applications.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.